1. Field of Invention
The present invention relates to a method for obtaining a signal of global positioning system (GPS), and more particularly, to a method for obtaining correct phase inversion points in a signal of GPS.
2. Related Art
A global positioning system (GPS) is a medium-range circular-orbit satellite navigation system. The GPS may provide accurate positioning, velocity measuring, and high-precision time standard for most of the areas (98%) on the surface of the earth. The GPS is developed and maintained by the U.S. Department of Defense for fulfilling the demands of continuously and accurately determining three-dimensional positions, three-dimensional movements, and time for a military user at any place on the globe or in the terrestrial space. The system includes 24 GPS satellites in the space, one master control station, 3 data upload stations, 5 monitor stations, and GPS receivers serving as user terminals on the earth. Only 4 satellites are needed at least to determine the position and altitude of the user terminal on the earth rapidly. The larger the number of satellites from which the user terminal may receive data, the more precise the decoded position is.
The GPS has features such as being free from weather conditions, a high global coverage rate (98%), and moveable positioning, and therefore, in addition to military applications, the GPS is also widely used for civilian navigation (such as plane navigation, ship navigation, and vehicle navigation) and positioning (such as vehicle guard and positioning of mobile communication devices), etc.
A satellite will broadcast 1 bit ephemeris data through a phase angle (180 degrees) of an inverted signal every 20 milliseconds. That is to say, in a satellite signal, a time interval between phase inversion points is a multiple of 20 milliseconds. But as the satellite orbits the earth, when the GPS is receiving the satellite signal sent by the satellite, the satellite signals received by the GPS may vary in intensity due to the positions of the satellite. For example, when the satellite is right above the GPS, the signal is intense, and when the satellite is close to the horizon, the signal will be relatively weak. Meanwhile, the satellite signal may also be interfered by other electromagnetic radiations, so that the GPS might suffer from bad reception. Thus, when the satellite signal is weak or the noises are intense, the satellite signal received by the GPS might have phase inversion points that are not multiples of 20 milliseconds due to the interferences. Accordingly, misjudgment actions occur when the GPS is resolving the satellite signal. A time for correct positioning is extended as it is required a repeated and time-consuming process of receiving the satellite signals all over again for positioning.